Duplication along curve with Mirror modifier
Instead of using the duplicate method post din your questions you can use an array modifier and a curve modifier.

1. Add Curve


Add -> Curve -> Bezier
move it (in my case I moved it along the positive x-axis)
Place the origin at the start of the curve path


select curve -> edit mode -> Select start point -> Shift S Cursor to selected -> Object Mode -> Ctrl Alt Shift C -> Origin to 3d cursor





2. 


Add a plane at the same location
Scale it down


edit mode -> S

Apply Location: Ctrl A Location




3. 

Select plane, add array modifier
Fit Type: Fit Curve
Relative Offset modify to move in the direction of the curve



4.

Add Curve modifier to plane



5.

Add mirror modifier





image description http://www.pasteall.org/pic/show.php?id=67294